WHat are the chances the coupling caps in an early Traynor YGL3 MK 3 are still performing to spec ?
If, as I expect, they should be replaced, is ther a particular make or type that should be considered ?
Coupling caps life ?
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Stevem
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Re: Coupling caps life ?
The only time they need to be replaced other then for possible tone improvements is if there leaking dc voltage down stream into the next circuit.
Of course this means they would have to have a high dc voltage on there other end as in the case of a plate decoupling cap.
Just unsolder the down stream end and measure what you have for DC.
A few millivolts is ok, I would say that more then 150 MV and that can start to be a issue.
Of course this means they would have to have a high dc voltage on there other end as in the case of a plate decoupling cap.
Just unsolder the down stream end and measure what you have for DC.
A few millivolts is ok, I would say that more then 150 MV and that can start to be a issue.
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Re: Coupling caps life ?
Usually, coupling capacitors of film, ceramic or mica construction have very long, to essentially indefinite life. The visual inspection is to look for physical damage, such as cracked epoxy coating where the leads enter the body of the device. That lets humidity in, which degrades the dielectric over time.
There are exceptions. There are known vintage caps that have a tendency to fail. The "yellow Astron" (blue-point from the color of the end seals) in Fender tweed amps, used a mylar type film which is unstable and chemically breaks down over time. A lot of cheap mica caps with no branding tend to fail too.
In older equipment you'll find troublesome things, like paper-in-oil and wax capacitors. These are pretty much suspect when you see them.
There are sometimes advantages to certain capacitor materials or construction, in certain applications.
Some ceramic dielectrics are piezoelectric. Some have a small rectifying effect, some can be non-linear. Some ceramic dielectrics are perfectly fine for audio coupling or filtering. Usually, you might want to pick NP0 or C0G ceramics. The other types may be ideal for power-supply decoupling or in RF filtering and so on, but perhaps not ideal in audio coupling.
Since I buy in hobby/repair quantities I'm not too worried about getting the price down to the smallest fraction of a penny per unit. I tend to pick polypropylene dielectric film caps for stock, since they are really good for what I usually use them for. Mylar (aka polyester) is perfectly good too, cheaper and smaller. For smaller caps under 1000pf, polystyrene has very good electrical qualities, but can be hard to find today. Appropriate ceramic and known-quality silver mica are perfectly fine.
There are exceptions. There are known vintage caps that have a tendency to fail. The "yellow Astron" (blue-point from the color of the end seals) in Fender tweed amps, used a mylar type film which is unstable and chemically breaks down over time. A lot of cheap mica caps with no branding tend to fail too.
In older equipment you'll find troublesome things, like paper-in-oil and wax capacitors. These are pretty much suspect when you see them.
There are sometimes advantages to certain capacitor materials or construction, in certain applications.
Some ceramic dielectrics are piezoelectric. Some have a small rectifying effect, some can be non-linear. Some ceramic dielectrics are perfectly fine for audio coupling or filtering. Usually, you might want to pick NP0 or C0G ceramics. The other types may be ideal for power-supply decoupling or in RF filtering and so on, but perhaps not ideal in audio coupling.
Since I buy in hobby/repair quantities I'm not too worried about getting the price down to the smallest fraction of a penny per unit. I tend to pick polypropylene dielectric film caps for stock, since they are really good for what I usually use them for. Mylar (aka polyester) is perfectly good too, cheaper and smaller. For smaller caps under 1000pf, polystyrene has very good electrical qualities, but can be hard to find today. Appropriate ceramic and known-quality silver mica are perfectly fine.
Re: Coupling caps life ?
+1 for nuke's comment. Coupling caps has semi-infinite life. They only die when a transient voltage punctures the insulation, heat cooks them til they short, or they get mechanically abused.
Those things do happen sometimes, but they are rare. these kinds of caps do not have a built-in decay mechanism like electrolytics do.
Those things do happen sometimes, but they are rare. these kinds of caps do not have a built-in decay mechanism like electrolytics do.
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